Electrolytic silver recovery from used photographic solutions is a common way to extend the life of such solutions.
An apparatus for the electrolytic recovery of silver from solutions containing silver is known from United States patent U.S. Pat. No. 5,378,340 (Michiels et al. assigned to Agfa-Gevaert NV) issued 3 Jan. 1995. The apparatus comprises an electrolytic cell including: a housing; an anode having an exposed anode portion within the housing; and a cathode having an exposed cathode portion located within the housing and encircling the anode. In use silver from the silver containing solution is deposited on the face of the cathode which is directed towards the anode. After the cell is operated for some time, the cathode is removed from the cell and replaced.
In a known method of removing silver from silver-containing aqueous liquids, the liquid to be treated is pumped into the electrolytic cell and electrical power is fed to the anode and the cathode to cause silver to be deposited on the cathode. The cathode is usually removable, and after a certain amount of silver has built up thereon, the cathode is removed and replaced. In the cell described in U.S. Pat. No. 5,378,340, the electrical connection to the cathode is below the liquid level in the cell, so that deposits may form on this connection. This leads to unpredictability in the electrical energy fed to the cell, making control of the process difficult. Furthermore, a reference electrode is placed in a side arm of the housing and in order for this electrode to accurately reflect the condition of the bulk of the liquid in the cell, it was necessary to form the cathode with holes there-through. Such holes also contribute to good circulation of the electrolyte through the cell. Not only does the formation of these holes constitute an additional manufacturing step, the holes result in uneven deposition of silver on the cathode. Also, the presence of gas bubbles in the liquid, which bubbles may be seeded for example at the surface of the cathode, may cause non-uniform deposition of silver to occur.
We are aware of United States patent U.S. Pat. No. 4,439,300 (Kenneth R Houseman/General Dental Inc.) which describes a device for collecting silver from photographic solutions in which a vortex is generated to promote increased electrolytic action. The device includes an anode in the form of a cylindrical tube, the upper end of which leads to an outlet passage in an upper part of the device. The disclosure in this patent fails to address the problem of de-aerating the device. Furthermore, the device described in U.S. Pat. No. 4,439,300 cannot be filled with liquid from empty in a simple and convenient manner.
We are also aware of French patent application FR 2270345 (Eastman Kodak Company) which describes a process for the recovery of silver in an electrolytic cell in which the solution is circulated by reduced pressure. The disclosure in this patent application fails to address the problem de-aerating the cell.